aerodynamics

A model of a supersonic transport being tested in
a wind tunnel at NASA's Langley Research Center in July 1973. Credit:
NASA.

Aerodynamics is the branch of physics dealing with the flow of air or other
gas around a body in motion relative to it. Aerodynamic forces depend on
the body's size, shape, and velocity; and on the density,
compressibility, viscosity, temperature,
and pressure of the gas. At low velocities,
flow around the body is streamlined (see streamlining)
or laminar, and causes low drag; at higher
velocities turbulence occurs, with fluctuating
eddies, and drag is much greater. Streamlined objects, such as airfoils,
are designed to maintain laminar flow even at relatively high velocities. Pressure impulses radiate at the speed
of sound ahead of a moving body; at supersonic velocities these impulses pile up, producing a shock
wave, and, in some cases, an associated sonic
boom.

In airplane design all of these factors
must be considered. In normal cruising flight all the forces acting on a
plane must balance (see forces of flight).
The lift provided by the wings must equal the
aircraft's weight; the forward thrust of the engines must balance the forces of drag. Lift occurs because the
wing's upper surface is more convex, and therefore longer, than the lower
surface. Air must therefore travel faster past the upper surface than past
the lower, which leads to reduced pressure above the wing.

Related terms include: (1) aerodynamic efficiency, which
is the efficiency with which an airfoil uses the aerodynamic forces acting
on it; in particular the ration of lift to drag; (2) aerodynamic
heating, which is heating produced by friction when flying at high
speed through an atmosphere; and (3) aerodynamic vehicle,
which is a vehicle such as an airplane or glider capable of flight when
moving through an atmosphere by generating aerodynamic forces.